Musnad Ahmad 788:204

Bahz narrated to us, Hammad—meaning Ibn Salamah—narrated to us, Abu Imran al-Jawni narrated to us, from Abu Bakr ibn Abi Musa, from his father, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Rejoice and give glad tidings to the people. Whoever says: 'There is no god but Allah,' being truthful in it, will enter Paradise." So they went out to give glad tidings to the people, and Umar (may Allah be pleased with him) met them, so they gave him the glad tidings. He sent them back. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Who sent you back?" They said: "Umar." He said: "Why did you send them back, O Umar?" He said: "If [you do that], the people will rely [solely] upon it, O Messenger of Allah."